Chip attach frame

ABSTRACT

A chip attach frame is used to align pins of an integrated circuit chip with pads on a chip carrier. A frame block has a socket defining two alignment edges that form a reference corner. The chip is lowered into the socket, and the chip carrier is inclined while it supports the frame block and chip until the chip moves under force of gravity to the reference corner. Once located at the reference corner, the chip position is carefully adjusted by moving the frame block in the x- and y-directions until the pins are aligned with the pads. The frame block is spring biased against movement in the x- and y-directions, and the position of the frame block is adjusted using thumbscrews. A plunger mechanism can be used to secure the integrated circuit chip in forcible engagement with the chip carrier once the pins are aligned with the pads.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of copending U.S. patent applicationSer. No. 14/017,311 filed Sep. 4, 2013, which is a divisional ofcopending U.S. patent application Ser. No. 13/396,207 filed Feb. 14,2012, now U.S. Pat. No. 8,535,956.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention generally relates to the interconnection ofelectronic devices such as integrated circuit chips, and moreparticularly to a method of providing a temporary attachment between anintegrated circuit chip and a carrier or package for testing.

Description of the Related Art

Integrated circuits are used for a wide variety of electronicapplications, from simple devices such as wristwatches, to the mostcomplex computer systems. A microelectronic integrated circuit (IC) chipcan generally be thought of as a collection of logic cells formed on asemiconductor substrate (e.g., silicon), with electricalinterconnections between the cells. An IC may include a very largenumber of cells and may require a large number of external metalliccontacts to serve as input or output pins.

As integrated circuit designs become more complex and the size ofintegrated circuit chips continues to shrink, pin densities grow and itbecomes increasingly more difficult to interconnect the chip to externalcircuitry. Chips are commonly attached to a main system substrate suchas a printed circuit board (PCB) using a carrier or package which fansout the connections from the external pins of a chip to wires on thePCB. FIG. 1 illustrates a typical chip assembly 10 which includes an ICchip 12, a chip carrier 14, a PCB 16, and miscellaneous components suchas capacitors 18. These various elements may be electrically coupledusing surface-mount connections with controlled collapse chip connection(C4) solder ball arrays. IC chip 12 is connected to package 14 which isin turn connected to PCB 16. Package 14 and PCB 16 both have multiplehorizontal layers interconnected by vertical vias. A single layer maycontain multiple planes, i.e., some for wiring and others for anelectrical ground plane or a power plane. A given plane in package 14may have multiple connections to the top and bottom surfaces to coupleground or power planes of IC chip 12 to ground or power planes of PCB16. For state-of-the-art designs, it may be necessary to connecthundreds of chip pins to respective carrier pads.

SUMMARY OF THE INVENTION

The present invention is directed to a chip attach frame for attachingan integrated circuit chip to a chip carrier using a frame block havinga generally rectangular socket with two adjacent alignment edges formingan orthogonal reference corner, the socket being larger than theintegrated circuit chip and located such that pads of the chip carrierare accessible through the socket when the frame block is placed on thechip carrier, the frame block being movable in at least first and secondnon-parallel directions along the chip carrier, first means foradjusting a position of the frame block relative to the chip carrieralong the first direction, second means for adjusting the position ofthe frame block relative to the chip carrier along the second direction,and plunger means for securing the integrated circuit chip in forcibleengagement with the chip carrier. In some embodiments the plunger meansincludes a pressure plate which contacts an upper surface of theintegrated circuit chip, one or more elastomeric strips interposedbetween the pressure plate and an upper surface of the frame block, aplunger block which drives the pressure plate, and actuation means forforcing the plunger block toward the integrated circuit chip. The chipattach frame can have temperature control elements located in theplunger block. The chip attach frame can further have a plurality ofpositioning blocks attached to an upper surface of the chip carrierwhich engage channels formed in a lower surface of the frame block. Atest board can be operatively attached to the chip carrier.

The above as well as additional objectives, features, and advantages ofthe present invention will become apparent in the following detailedwritten description.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be better understood, and its numerousobjects, features, and advantages made apparent to those skilled in theart by referencing the accompanying drawings.

FIG. 1 is a cross-sectional side view of a conventional integratedcircuit chip assembly including an integrated circuit chip which isconnected to a printed circuit board by means of a chip carrier;

FIG. 2 is an exploded perspective view of one embodiment of a chipattach frame constructed in accordance with the present invention whichis used to align pins of the integrated circuit chip with contact padsof a carrier;

FIG. 3A is a top plan view of one embodiment of a chip attach frameconstructed in accordance with the present invention showing theintegrated circuit chip loosely placed (unaligned) within an alignmentsocket of the chip attach frame;

FIG. 3B is a pictorial illustration of the chip attach frame of FIG. 3Adepicting inclination of the frame to allow the integrated circuit chipto be pulled by gravity toward a reference corner of the alignmentsocket;

FIG. 3C is a top plan view of the chip attach frame of FIG. 3A showinghow the integrated circuit chip has moved under influence of gravityinto forcible contact with alignment edges of the alignment socket; and

FIG. 4 is a cross-sectional side view of another embodiment of a chipattach frame constructed in accordance with the present inventionillustrating a plunger mechanism to temporarily maintain the integratedcircuit chip in contact with the carrier once the chip pins and carrierpads have been aligned.

The use of the same reference symbols in different drawings indicatessimilar or identical items.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

It is often necessary to connect an IC chip to a carrier temporarily,such as for testing purposes. The testing may be desired for qualityassurance at the factory, or for a chip that has already been deployedin the field but has suffered some defect. It is preferable to attachthe chip to the test carrier without a permanent (solder) connection sothat the chip may easily be removed from the test carrier and thenplaced into operation. It can be particularly advantageous to providesolderless connections in situations where for example one chip of amulti-chip module has been removed from the final product to diagnose afailure indication. In these situations when the chip underinvestigation is soldered to a test carrier, the diagnostic system oftenreturns a “No Defect Found” result because of unpredictable self-healingeffects that can arise during the hot solder process step.

One problem associated with temporary chip attachment for testingpurposes is the alignment of the chip pins with the carrier pads. Thereis very little tolerance in this alignment as the distance betweenadjacent chip pins (pitch) may be as small as 0.5 millimeters. Theproblem is exacerbated by saw cut variations in fabrication of the chipwhich can lead to inconsistent chip lengths or widths. Such variationsprevent alignment systems from using predefined measurements orplacement of the chip. One solution to this problem is the solderlesstemporary chip attachment method described in U.S. Pat. No. 6,127,254.According to that method, two thin pieces of polyimide foil are used asa placement stop for the pads, and the chip is moved until the pads arein contact with the foil edges. However, the foils are generally notreusable, and so add expense to the process. More complicated alignmentsystems similarly add excessive cost to the chip-carrier attachmentprocess.

In light of the foregoing, it would be desirable to devise an improvedmethod of aligning a chip for temporary attachment to a carrier such asfor testing purposes. It would be particularly advantageous if themethod could easily accommodate variations in die cut size whileretaining ease of use and low cost. The present invention achieves theseobjects by using gravity to pull a chip to a reference point or cornerinside an alignment socket of a chip attach frame which overlies a testcarrier. The position of the chip attach frame can then be easilyadjusted along x- and y-directions to properly align the pins of thechip to the pads of the test carrier.

With reference now to the figures, and in particular with reference toFIG. 2, there is depicted one embodiment 30 of a chip attach frameconstructed in accordance with the present invention for aligning pins32 of an integrated circuit chip 34 with pads 36 of a test carrier 38.Chip 34 may be freshly cut out of a wafer and ready for factory testingor may be desoldered/detached from a product carrier for diagnosing afailure. Chip attach frame includes a frame block 31 preferably having athickness which is slightly smaller than the thickness of chip 34. Inthis implementation chip 34 has a rectangular shape and frame block 31accordingly has a generally rectangular opening or alignment socket 40which is slightly larger than chip 34, i.e., the width of alignmentsocket 40 is slightly greater than the width of chip 34, and the lengthof alignment socket 40 is slightly greater than the length of chip 34.This increased size of alignment socket 40 allows chip attach frame 30to accommodate variations in chip size due to saw cut tolerances whilestill retaining chip 34 in a confined space relative to carrier 38. Thesocket is located within the frame block such that the carrier pads areaccessible when the frame block is placed on the chip carrier.

Alignment socket 40 preferably has sloping shelves on at least twoopposing sides (at the top surface of frame block 31) to facilitatelowering chip 34 in socket 40 and raising chip 34 out of socket 40. Forcost reasons the illustrative implementation relies on manual placementof chip 34 in socket 40 but a mechanical picking device couldalternatively be used. Chip attach frame 30 further includes first means42 for adjusting the position of frame block 31 along a first directionrelative to carrier 38 and second means 44 for adjusting the position offrame block 31 along a second direction relative to carrier 38. In theillustrative embodiment wherein the chip is rectangular and the insideedges of alignment socket 40 are orthogonal, the first direction is anx-direction in a Cartesian coordinate system and the second direction isa y-direction in that system, i.e., the two directions are likewiseorthogonal. In this manner, when chip 34 is placed inside socket 40, theentire assembly (chip 34, frame block 31, and carrier 38) may beinclined whereby chip 34 slides under the influence of gravity towardsthe lowest inside corner (a reference corner) of socket 40. Once sopositioned, the adjustment means 42, 44 can be employed to refine thelocation of chip 34 until its pins 32 are aligned over pads 36 ofcarrier 38.

With further reference to FIGS. 3A-3C, the lower left corner ofalignment socket 40 (as chip attach frame 30 is viewed from the front)has been designated as the reference corner 46. FIG. 3A shows an initialplacement of chip 34 within socket 40, with slight gaps along each ofthe four sides of chip 34, i.e., in a generally unaligned state. Chipattach frame 30 and carrier 38 are further resting on a test board 48which is electrically interconnected with carrier 38 and is part of thetesting/diagnostic equipment used to evaluate chip 34. The assembly istilted forward and to the left as depicted in FIG. 3B, whereupon gravitypulls chip 34 toward reference corner 46. The resulting view seen inFIG. 3C shows chip 34 forcibly abutting the inside alignment edges ofsocket 40 that together form reference corner 46. The assembly does notneed to be raised from the horizontal orientation to a fully verticalorientation (plumb), only sufficiently tilted such that the force ofgravity overcomes the frictional engagement of the lower chip surfacewith the upper carrier surface to allow the chip to slide within socket40.

In this state (with chip 34 positioned against reference corner 46),chip 34 may still be unaligned, that is, the pins of the chip may notyet be appropriately positioned for contact with the pads of thecarrier. If the chip is not aligned, the adjustment means are used topush chip 34 in the appropriate direction(s). In the illustrativeembodiment the x-direction adjustment means includes a first thumbscrew42 a in forcible contact with the left side of frame block 31 and afirst spring 42 b in forcible contact with the right side of frame block31. Spring 42 b biases the position of block 31 to the left, andthumbscrew 42 a is used to force frame block 31 to move to the rightagainst that spring bias. The y-direction adjustment means similarlyincludes a second thumbscrew 44 a in forcible contact with the frontside of frame block 31 and a second spring 44 b in forcible contact withthe back side of frame block 31. Spring 44 b biases the position offrame block 31 to the front, and thumbscrew 44 a is used to force frameblock 31 to move to the back against that spring bias. The thumbscrewsand springs can be mounted on backstops affixed to test board 48. Whilethe thumbscrews and springs provide a simple way to effectuate fineadjustment of the chip position, those skilled in the art willappreciate that other means may be used, such as electro-mechanical,piezo, stepping motor, etc. Verification of proper alignment can beachieved using an optical system or observing electrical contact resultsbetween the pins and the pads via the test system.

While socket 40 is illustrated as being rectangular, it could have othershapes, particularly if the chip is not rectangular. For example, if achip were hexagonal in shape, the socket could be hexagonal to match theshape, or could be triangular. It is also not necessary to have a socketthat is completely enclosed; only two edges are necessary to form thereference corner and to provide adjustment in two different directions.Moreover, in the illustrative embodiment the adjustment directions areparallel with the respective alignment edges of the sockets, but theadjustment directions can be oblique to the alignment edges. It shouldalso be apparent from the foregoing that the alignment edges do not haveto be orthogonal.

The invention may be further enhanced by providing a mechanism tosecurely retain the chip in temporary contact with the carrier once thepins/pads have been aligned. FIG. 4 illustrates another embodiment 30′of a chip attach frame constructed in accordance with the presentinvention which additionally includes a plunger mechanism that maintainspositive contact between the pins and pads. Chip attach frame 30′ has atop plate or beam 50 attached to the same backstops that support one ofthe sets of thumbscrews and biasing springs. Another thumbscrew 52passes through beam 50 and is used to actuate a plunger spring 54 whichis attached to a plunger block 56. Plunger block 56 rests atop apressure plate 58 which in turn rests atop chip 34. The bottommost sideof plunger block 56 may extend beyond pressure plate 58 for cases ofreduced chip height as indicated by the dashed lines. Rubber(elastomeric) strips 60 are interposed between pressure plate 58 and theupper surface of frame block 31 in case the plunger force is notuniform, for example due to a non-planar chip backside. Non-horizontalmovement of the plunger downwards will cause uneven pressure on one sideof the chip earlier than the other side. As a result, the generallydownward force vector of the plunger can include a small sideways forcevector. This sideways force vector can lead to displacement of the chip,i.e., causing new misalignment of the pads/pins. The rubber stripscompensate for any uneven pressure and level the plunger block face asit impacts the chip backside, which helps avoid cracking of the chip.

Plunger block 56 can be lowered against chip 34 while chip attach frame30′ is in the inclined position with chip 34 lying against referencecorner 46 under the force of gravity, and aligned by the adjustmentmeans. Once the plunger mechanism is actuated and the chip is secured inplace by the pressure plate, the assembly can be returned to ahorizontal orientation. The plunger mechanism can alternatively beactuated after the assembly is horizontal but this may introduce moreerror in the initial position of the chip.

In this embodiment the plunger mechanism also includes a temperatureregulation system which allows testing of the chip under differenttemperature conditions. Accordingly, no rubber layer is interposedbetween pressure plate 58 and chip 34 to ensure direct thermal transferbetween the pressure plate and the chip. The temperature regulationsystem may include elements 62 embedded in plunger block 56. Theelements may be used to heat the chip, cool it, or both, in a controlledfashion. For example, elements 62 may be heating elements that rely onJoule heating supplied by an electrical power source, or coolingelements that carry a coolant fluid such as liquid nitrogen supplied bya pump. The plunger mechanism may further have automated pressurecontrol using, e.g., a pneumatic system.

FIG. 4 also illustrates positioning blocks or bosses 64 which can beattached to or formed on the upper surface of test carrier 38 to keepframe block 31 in a generally central location by means of correspondingchannels cut into the bottom surface of frame block 31. Different frameblocks 31 can be provided for different chip sizes, i.e., havingdifferent socket sizes. A single chip attach frame can support multipletest carriers 38 by having multiple sockets, in which case multipleplunger mechanisms (including the rubber strips) can also be providedfor each chip location.

The components of a chip attach frame according to various embodimentsof the present invention may be constructed of any durable material.Frame block 31, plunger block 56, and pressure plate 58 are preferablyconstructed of a metal with an appropriate heat transfer coefficient.Rubber strips 60 are preferably a polymer, i.e., an elastomer whoseglass-liquid transition is below the operational temperature of the testsystem; different rubber strips could be used depending on the plannedtesting temperatures. The rubber strips can also be used to fix thepressure plate in the horizontal direction and thereby avoiddisplacement of the chip as the plunger is actuated. To this end, thetop and/or bottom surfaces of the rubber strips could be covered ortreated with special materials to increase the coefficient of friction.The rubber strips can for example be adhered to the top of the chipattach frame. The rubber material is most preferably sufficiently hardto avoid cracking of the chip substrate, e.g., silicon, when therequired force of the plunger is applied. The total required force for areliable contact is a function of the number of contact points betweenthe chip and the carrier. The dimensions of the chip attach frame mayvary considerably depending upon the particular application. In anexemplary embodiment adapted for a 25 mm×25 mm central processing unitchip, frame block 31 is approximately 35 mm×35 mm×5 mm.

The present invention can accordingly provide a quick and easy methodfor alignment and attachment of a chip to a carrier, resulting in a muchshorter turn around time for testing/diagnostics. Because the method canbe implemented without solder connections between the pins and pads, italso avoids “No Defect Found” situations that can arise fromself-healing effects during a hot solder process step. The inventionfurther accommodates individual saw cut tolerances in the chips withoutsignificant expense.

Although the invention has been described with reference to specificembodiments, this description is not meant to be construed in a limitingsense. Various modifications of the disclosed embodiments, as well asalternative embodiments of the invention, will become apparent topersons skilled in the art upon reference to the description of theinvention. For example, while the invention is particularly useful intemporary chip attachment, it is not constrained to temporary attachmentand could be used for permanent attachment wherein solder balls areprovided on the carrier and the chip/carrier assembly is passed througha furnace to melt with solder balls while still aligned by the chipattach frame. Similarly, the invention is not limited to testingpurposes but can also be employed for functional usage of a chip/carrierpackage. It is therefore contemplated that such modifications can bemade without departing from the spirit or scope of the present inventionas defined in the appended claims.

1.-13. (canceled)
 14. A chip attach frame for attaching an integratedcircuit chip to a chip carrier, comprising: a frame block having agenerally rectangular socket with two adjacent alignment edges formingan orthogonal reference corner, the socket being larger than theintegrated circuit chip and located such that pads of the chip carrierare accessible through the socket when said frame block is placed on thechip carrier, said frame block being movable in at least first andsecond non-parallel directions along the chip carrier; first means foradjusting a position of said frame block relative to the chip carrieralong the first direction; second means for adjusting the position ofsaid frame block relative to the chip carrier along the seconddirection; and plunger means for securing the integrated circuit chip inforcible engagement with the chip carrier.
 15. The chip attach frame ofclaim 14 wherein said plunger means includes: a pressure plate whichcontacts an upper surface of the integrated circuit chip; one or moreelastomeric strips interposed between said pressure plate and an uppersurface of said frame block; a plunger block which drives said pressureplate; and actuation means for forcing said plunger block toward theintegrated circuit chip.
 16. The chip attach frame of claim 15, furthercomprising temperature control elements located in said plunger block.17. The chip attach frame of claim 14, further comprising a plurality ofpositioning blocks attached to an upper surface of the chip carrierwhich engage channels formed in a lower surface of said frame block. 18.The chip attach frame of claim 14, further comprising a test boardoperatively attached to the chip carrier.